Semiconductor substrates and devices are becoming smaller and more dense with the evolution of new technologies. However, increases in circuit density produces a corresponding increase in overall manufacturing problems. These manufacturing problems must however be kept to a minimum in order for the semiconductor manufacturer to remain competitive. The semiconductor manufacturers are therefore constantly being challenged to improve the quality of their products by identifying and eliminating defects which produce defective parts or components. Whereas significant improvements are being made to eliminate systematic defects by reducing process variability. Process improvements alone are not sufficient to eliminate all the random defects which effect both yield and reliability. Historically, screening techniques have been employed to improve product failure rates to acceptable levels by culling out many of these random defects.
In their desire to improve their products the semiconductor manufacturers are constantly finding new ways and new techniques to improve or provide new products. It has been found that for some applications one could make a ceramic carrier or substrate having a cavity and then have the semiconductor chip placed inside the cavity and secured to the semiconductor substrate. These semiconductor substrates are often referred to as modules. These modules could be made from a single ceramic layer or green sheet forming a single layer ceramic module or a plurality of ceramic layers forming a MLC (multilayer ceramic) module.
These MLC modules having single or multiple cavities are normally used in the electronic industry to package high performance integrated circuits or chips. These high performance integrated circuits chips have large number of external input/output points, such as, pads, solder balls, to name a few, and these chips have a very high power dissipation. In order to accommodate such high performance chips the MLC substrate or module also have to provide high number of external input/output points, such as, pads, pins, to name a few, and also be able to handle the very high power dissipation that is being generated both from the module as well as the chip.
The single or multiple cavities in a MLC module are normally formed during the lamination process using typically a hard or a soft insert as a plugs. This plug in turn prevents the collapse or deformation of the stacked green ceramic body or sheet during lamination. This method of producing single or multiple cavities requires machining of the inserts with high precision and with high level of surface finish.
Inherently, the cost of such inserts or plugs is very high. Additionally, these inserts or plugs do not provide the flexibility of using the same inserts for cavities of various shapes and sizes. Furthermore, placing these inserts and then subsequently removing them is an expensive process and many times this could lead to the delamination of the ceramic green sheets. Another drawback with these solid inserts is the need to clean them prior to every use to avoid the paste pull-outs or damage to the green ceramic layers.
Another method of producing these single or multiple cavities in the MLC module would be to machine the cavities after the green sheets have been laminated, but this would not be a cost effective way of producing parts in a high volume manufacturing operation.
It is also possible to form cavities in the MLC module with no inserts. This could be done for cases where the lamination conditions are such that there is no resulting deformation in green ceramic sheet or body. In these cases, typically, the lamination pressures are very low and the green sheet formulation is such that the dimensional control of products is achieved by altering the sintering process. However, in high volume manufacturing operation, tailoring the green sheet formulation and developing a sinter cycle for every product would be cost prohibitive and time consuming. Besides, this approach typically needs an adhesive between layers and multiple lamination steps to achieve the end result. Thus, some of the problems associated with this low pressure lamination process are that no process window for dimensional control is available for the sintered body. Delamination of the ceramic layer could happen in sintering due to the removal of the adhesive and the density gradients in the starting structure that are normally present could result in poor substrate dimensional control. Furthermore, there could be substantial increase in stacking and lamination cost and limitation in metal loading on the green sheets to have effective green sheet bonding.
In order to better understand the related art, the art could be broadly grouped into three general categories. The first being the art that is directed to shaping of an object by material deformation. The second being the art that deals with formation of non-planar and non-deformable objects by using cavity fills. While, the third art group could be one that deals with planar objects.
The following references could be classified under the first group which deals with deformation and shaping of a planar object.
U.S. Pat. No. 4,946,640 (Nathoo) shows that the deformation of the material could be accomplished by using a mold of desired contours. In this patent air is used as the lubricant and release agent to separate the preformed material from the mold.
U.S. Pat. No. 5,108,532 (Thein) shows that the deformation of a planar object could be accomplished by applying air on one side and vacuum on the other side.
The following references could be generally classified under the second group which deals with non-planar and non-deformable objects. However, this art group could be further divided into two unique sub-groups, where the first sub-group would be dealing with a non-planar lamination using solid insert, and the second sub-group would be dealing with a non-planar and non-deforming lamination with no inserts.
The following references could be classified in this first sub-group of non-planar lamination using solid insert.
U.S. Pat. No. 4,024,629 (Lemoine) shows that fugitive paste could be used as a cavity fill to prevent deformation during lamination.
U.S. Pat. No. 4,680,075 (McNeal) shows that a thermoplastic plug could be used to fit snugly inside a cavity as a cavity fill.
IBM Technical Disclosure Bulletin, Phillips, Vol. 16, No. 11, Pages 3559 (April 1974) shows that a metal insert could be used to fill a cavity.
The following references could be classified in the second sub-group of non-planar and non-deforming lamination with no inserts.
Both, U.S. Pat. No. 4,636,275 (Norell) and No. 4,824,509 (Tonoki) teach how to laminate a non-planar object with a non-deforming cavity by using a bag/bladder/elastomer separating the liquid applying the force from the object to be laminated.
Finally, following is a reference that could be classified in the third group that deals with planar objects.
U.S. Pat. No. 4,734,155 (Tsunoda) teaches the use of a mechanical device to apply pressure to the lamination plates.
All cavity lamination methods described above can also be further classified into three general groups depending on the manner in which they interact with the body being laminated. The first group could be the use of solid inserts to fabricate cavities that are planar with the laminating surface since the inserts themselves need to conform to the laminating surface.
The second group could be the use of a fluid and membrane to laminate the multilayer ceramic assembly that does not have to conform to any surface since the fluid applying the pressure distributes the force uniformly over the cavity surface.
The third group could include the use of fugitive paste which does conform to the laminating surface wherever there is no fugitive paste; but it does not need to conform to the laminating surface within the cavity region since the paste can have low viscosity relative to the viscosity of the multilayer ceramic body during the lamination process.
One of the problems that arises when fabricating cavities using one of the first group methods is the removal of the solid object or insert that was used to prevent the cavity collapse prior to the sintering process. Removal of a solid object from an unsintered ceramic body can cause layer separation or other mechanical damages to the unfired ceramic. Also, since the solid plug or insert used does not compress itself during the lamination process as the unfired ceramic body does, it is possible to create damage to the metallurgy or the ceramic module prior to sintering.
The use of a liquid and a membrane to laminate a cavity as expected from the methods described in the second group can eliminate most of the mechanical problems experienced with the first group methods; however, there are other new problems associated with the use of the second group method. The first being that it is possible to rupture the membrane and get the fluid to contaminate the multilayer ceramic assembly. And, the second is that it is difficult to maintain planarity on the cavity surface when the metal distribution within the package is not uniform prior to the lamination stage.
The present invention however solves all of the above mentioned problems by providing a cavity formation method at lamination that maintains planarity to the laminating surface during the laminating process while simultaneously preventing the cavity from collapsing without the use of a solid insert within the cavity volume.
The present invention primarily deals with the formation of cavities in multi-layer ceramic (MLC) modules or substrates, without the use of hard (like metal) or soft (like polymer) inserts during the lamination process.
Basically, this invention teaches a method to laminate a non-planar object with no cavity deformation by:
(a) applying force mechanically to the object to be laminated and simultaneously, by PA1 (b) applying pressure to the cavity volume using high pressure fluid, such as gas, to balance the deformation forces generated by the mechanically applied forces. PA1 (a) placing at least one ceramic layer having at least one cavity over a first plate, PA1 (b) placing at least one cavity forming membrane over at least a portion of said at least one cavity, PA1 (c) placing a second plate having at least one opening over at least a portion of said at least one cavity, PA1 (d) putting pressure on at least a portion of said second plate so that said first and second plates come closer and simultaneously pressurizing said at least one cavity in said ceramic layer with at least one gas to counter the pressure from said second plate, thereby forming a substrate with a cavity without the use of an insert. PA1 (a) placing at least one ceramic layer having at least one cavity over a first plate, PA1 (b) placing at least one cavity forming membrane having at least one opening over at least a portion of said at least one cavity, such that at least a portion of said opening in said membrane is over at least a portion of said cavity, PA1 (c) placing a second plate having at least one opening over at least a portion of said at least one opening in said membrane, PA1 (d) putting pressure on at least a portion of said second plate so that said first and second plate come closer, and simultaneously pressurizing said at least one cavity in said ceramic layer with at least one gas to counter the pressure from said second plate, thereby forming a substrate with a cavity without the use of an insert.
Thus, this invention is unique and different than the methods well known in the prior art.